The present invention relates to shackle mechanisms and is particularly concerned with handcuffs of the so-called "arrest" type.
The conventional "arrest" handcuff comprises a pair of arcuate arms pivoted together at one of their respective ends and interengageable at their other respective ends so as collectively to encircle the wrist. One arm (which, for convenience, we shall refer to as the "locking arm") is formed with a series of ratchet teeth on its convex side at its end remote from the aforesaid pivot while the other arm (which, for convenience, we shall refer to as the "receiving arm") extends from a casing which houses a spring-biased pawl for engagement with the ratchet teeth of the locking arm. The ratchet teeth and pawl are so arranged as to permit continuous 360.degree. rotation of the locking arm relative to the receiving arm in the direction which closes the wrist aperture defined by the arms but to prevent rotation of the locking arm relative to the receiving arm in the opposite direction when the ratchet teeth and pawl are in contact. When making an arrest, therefore, the receiving arm is placed against the wrist of the subject and the locking arm is swung into engagement therewith until it meets the obstruction of the now-encircled wrist; from this condition the locking arm is automatically prevented from withdrawal by virtue of the engagement of its ratchet teeth with the pawl of the receiving arm acting under the aforesaid spring bias. To release the cuff a simple "key" is provided to the proper authorities which when inserted into the pawl casing and turned in the appropriate direction engages the pawl to lift it out of engagement with the ratchet teeth against the action of its spring bias, thus to permit rotation of the locking arm away from the receiving arm.
Handcuffs operating on the above principle have been used for many years. Nevertheless, the existing forms of cuff still have certain drawbacks from the point of view of security. In particular, the absence of a secure key-recognition mechanism which must be operated before the pawl is lifted to release the locking arm means that these devices are relatively vulnerable to picking. It is true that in certain designs additional "deadlocking" means are provided which can be applied to block the lifting of the pawl if an additional manipulation is performed by the arresting officer after the cuff has been fitted to the subject's wrist--such as the slidable bolt disclosed in U.S. Pat. No. 1,161,562. However the mechanism shown in the above-mentioned specification is typical of such known devices in that the aforesaid bolt is arranged to be released by the same operation of the same simple "key" as is used to lift the pawl against its spring bias, and is unprotected by any proper key-recognition mechanism.